Free-radical polymerization is widely used to make many important commercial polymers, including polystyrene, acrylic polymers, polyvinyl chloride, and synthetic rubber. The high reactivity of ethylenic monomers often complicates the synthesis. Thus, many free-radical polymerizations are performed in suspensions, emulsions, or solutions using solvents and chain-transfer agents to regulate reaction rate and/or polymer molecular weight. Examples are the processes now used to make resins for electrophotographic toners.
Ordinary, black toner resins are high-molecular-weight copolymers of styrene with butadiene or acrylates made by suspension or emulsion polymerization techniques. Because these copolymers can be difficult to pulverize, can adhere poorly to paper, can have poor anti-offset or fusion properties, and can have poor abrasion resistance, newer and more-expensive resins (requiring an even more complex synthesis) have evolved. The newer systems use mixtures of high and low-molecular-weight polymers, or they include a second, low-molecular-weight component (i.e., they are bimodal polymers). While the processability and performance of the new resins are better than that of ordinary toner resins, preparing the new resins can be complicated and expensive.
Allyl alcohol and its derivative monomers (e.g., allyl ethers, allyl esters, and alkoxylated allylic alcohols) readily copolymerize with ethylenic monomers (e.g., styrene or methyl methacrylate) to give low-molecular-weight, hydroxy-functional resins. We have described these hydroxy-functional resins and ways to make them in several recently issued patents (see, e.g., U.S. Pat. Nos. 5,475,073, 5,525,693, and 5,571,884). As we noted there, the allyl monomer helps to control the molecular weight of the polymer and eliminates the need for the solvents and chain-transfer agents commonly used in free-radical polymerizations. The resins usually have low molecular weights (Mn=about 1000 to 5000), narrow molecular weight distributions (Mw/Mn&lt;3), and evenly distributed hydroxyl groups. As we explained earlier, these resins are valuable reactive intermediates for making high-performance coatings and other thermoset polymers. The resins are crosslinked with melamines, polyisocyanates, epoxies, and other crosslinkers to give useful thermosets (see U.S. Pat. Nos. 5,534,598 and 5,480,943).
The crosslinkable allylic copolymer resins described above are ideal for many coatings. Unfortunately, however, they are not so valuable as toner resins, for which higher molecular weights and broader molecular weight distributions are generally needed. Making useful high-molecular-weight polymers with broad molecular weight distributions is not always easy, particularly when allylic monomers are used. While crosslinking monomers such as divinylbenzene (DVB) are commonly used to increase molecular weight in free-radical polymerizations, they can normally be used only in minor concentrations. At high enough DVB levels, excessive crosslinking occurs, and a gelled reaction mixture can result.
Applications other than toners could also benefit from new allylic copolymer resins. Examples include thermoset polymers (which normally include, in addition to the resin, a crosslinker and other optional components such as fillers, thickeners, pigments, and other additives), thermoplastics (which can benefit from plasticizers that can react into the polymer network), and coatings (especially for controlling rheology).
In sum, there is a continuing need for new allylic copolymer resins and ways to make them. Preferably, the re sins would offer processing and performance benefits--particularly in the areas of electrophotographic toners--that are not currently available from the low-molecular-weight, hydroxy-functional acrylic resins described above. A preferred process would be inexpensive and simple to practice, and would retain the benefits of earlier processes for making resins from allyl monomers (e.g., good control over molecular weight build even without a solvent or chain-transfer agent). Finally, a preferred process would provide resins having high molecular weights and broad molecular weight distributions but would avoid excessive crosslinking and reactor gellation.